JP6035330B2 - Non-natural equilibrium supply system for injection molding equipment - Google Patents

Description

  The present invention relates to an apparatus and method for injection molding, and more particularly to an apparatus and method for producing injection molded parts at a low constant pressure.

  Injection molding is a technique commonly used for mass production of parts made of meltable materials, most commonly parts made of thermoplastic polymers. During the repeated injection molding process, a plastic resin (mostly in the form of small beads or pellets) is introduced into an injection molding machine that melts the resin beads under the application of heat, pressure, and shear. The molten resin is forcibly injected into a mold cavity having a specific cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape essentially replicating the mold cavity shape. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to the flow path by a gate that guides the flow of molten resin into the cavity. The molded part may have one or more gates. Large parts typically have two, three, or more gates to reduce the flow distance that the polymer must travel to fill the molded part. One gate or multiple gates per cavity may be located anywhere on the part shape, for example, any one that is essentially circular or formed with an aspect ratio of 1.1 or greater It may have a cross-sectional shape. Thus, a typical injection molding procedure consists of (1) heating a plastic in an injection molding machine and flowing it under pressure, and (2) a mold cavity defined between two closed mold halves. Injecting molten plastic into the inside, (3) Cooling and hardening the plastic in the cavity under pressure, (4) Opening the mold half and removing the part from the mold, including four basic operations .

  Molten plastic resin is injected into the mold cavity and the plastic resin is pushed into the cavity by an injection molding machine until the plastic resin reaches a position in the cavity furthest from the gate. The length and wall thickness of the part thus obtained is a result of the shape of the mold cavity.

  Although it may be desirable to reduce the wall thickness of the injection molded part to reduce the plastic content of the final part, and hence the cost, it is particularly important to reduce the wall thickness using conventional injection molding processes. When designing for wall thicknesses of less than 10, 3, and 1.0 millimeters, it can be an expensive and not easy task. When liquid plastic resin is introduced into the injection mold by conventional injection molding methods, the material adjacent to the cavity walls immediately begins to “solidify” or solidify and harden. As the material flows through the mold, a boundary layer of material is formed on the sides of the mold. As the mold continues to be full, the boundary layer continues to thicken, thereby closing the path through which the material flows and preventing further material from flowing into the mold. The solidification of the plastic resin on the mold wall worsened when the mold was cooled, and techniques were used to reduce the cycle time of each part and increase the machine throughput.

  There is also a need to design parts and corresponding molds so that the liquid plastic resin flows from the thickest region to the thinnest region. Increasing the thickness of a particular area of the mold ensures that sufficient material flows into the area where strength and thickness are required. The need for this “thick to thin” flow path makes the use of plastic inefficient, and as a result, additional material must be molded into parts where material is not needed, so injection molding The parts cost of the parts manufacturer increases.

  One way to reduce the thickness of the part is to increase the pressure of the liquid plastic resin when it is introduced into the mold. By increasing the pressure, the molding machine can continue to inject liquid material into the mold until the flow path is closed. However, increasing pressure tends to reduce both cost and capacity. As the pressure required to mold the components increases, the molding machine must be robust enough to withstand the additional pressure and is generally expensive. Manufacturers may have to purchase new equipment to accommodate high pressures. Therefore, reducing the wall thickness of a given part significantly increases the capital costs produced by conventional injection molding techniques.

  Furthermore, when the liquid plastic material flows into the injection mold and freezes rapidly, the polymer chain maintains the high degree of stress that was present when the polymer was in liquid form. The frozen polymer retains a higher level of flow-induced orientation when the molecular orientation is fixed in the part, resulting in a frozen stress state. Such “residual molding” stress can distort or dent subsequent molding, causing the production of parts with low mechanical properties and low resistance to chemical exposure. Low mechanical properties are particularly important for the control and / or minimization of injection molded parts such as thin walled tubes, integral hinge parts, and closures.

  To avoid some of the above disadvantages, many conventional injection molding operations use shear-thinning plastic materials to improve the flow of plastic materials into the mold cavities. When shear-thinning plastic material is injected into the mold cavity, the shear force generated between the plastic material and the mold cavity wall tends to reduce the viscosity of the plastic material, thereby causing the plastic material to Flow more freely and easily into the mold cavity. As a result, it is possible to fill thin wall parts fast enough to prevent the material from freezing before the mold is completely filled.

  The reduction in viscosity is directly related to the magnitude of the shear force generated between the plastic material and the delivery system and between the plastic material and the mold cavity wall. Therefore, manufacturers of these shear thinning materials and operators of injection molding systems have driven injection molding pressures higher to increase shear forces and thus reduce viscosity. Typically, an injection molding system injects a plastic material into a mold cavity at a melt pressure of 103.4 MPa (15,000 psi) or higher. Manufacturers of shear-thinning plastic materials teach injection molding operators to inject plastic material into the mold cavity above the minimum melt pressure. For example, polypropylene resins are typically processed at pressures above 41.4 MPa (6,000 psi) (polypropylene resin manufacturers' recommended range is typically above 41.4 MPa, about 103 Less than 4 MPa (6,000 psi to about 15,000 psi)). Resin manufacturers do not recommend exceeding the upper limit of the range. Press machine manufacturers and processing engineers typically typically exceed 103.4 MPa (15,000 psi) to derive maximum thinning and better flow properties from plastic materials In order to achieve maximum shear thinning, it is recommended to treat shear thinning polymers at the upper end of the range or significantly higher. The shear-thinning thermoplastic polymer is generally processed in the range of greater than 41.4 MPa to about 206.8 MPa (6,000 psi to about 30,000 psi).

  Molds used in injection molding machines must be able to withstand these high melt pressures. Furthermore, the material forming the mold must have a fatigue limit that can withstand the maximum cyclic stress for the total number of cycles that the mold is expected to perform during its lifetime. As a result, mold manufacturers typically form molds from materials with high hardness, typically greater than 30 Rc, more typically greater than 50 Rc. These hard materials are durable and equipped to withstand the high clamping pressures required to keep the mold components pressed against each other during the plastic injection process. These high hardness materials can also better withstand wear from repeated contact between the molding surface and the polymer flow.

High production injection molding machines that produce thin wall consumer products (ie, class 101 and class 102 molding machines) exclusively use molds that have a majority of molds made from high hardness materials. High production injection molding machines typically produce over 500,000 cycles per year. Industrial quality production molds are at least 500,000 cycles per year, preferably more than 1,000,000 cycles per year, more preferably more than 5,000,000 cycles per year, even more preferably per year It must be designed to withstand over 10,000,000 cycles. These machines have multiple cavity molds and complex cooling systems to increase production rates. High hardness materials can withstand repeated high pressure clamping operations more than lower hardness materials. However, high hardness materials, such as most tool steels, generally have a relatively thermal conductivity of less than 34.6 W / (m ^* K) (20 BTU / HR FT ° F), which allows heat to pass through the hard material. As it is transmitted from the molten plastic material, it results in a long cooling time.

  To reduce cycle time, a typical high production injection molding machine with a mold made from a hard material includes a relatively complex internal cooling system that circulates a cooling fluid within the mold. These cooling systems accelerate the cooling of the molded parts, thus allowing the machine to complete more cycles in a given time, which reduces the production rate and thus the total amount of molded parts produced. increase. In some classes 101 that exceed 100 or 2 million cycles per year, these molds may be referred to as “ultra-high productivity molds”. Class 101 molds that operate in presses of 400 tons or more are sometimes referred to in the industry as “400 class” molds.

  Another disadvantage of using high hardness materials for the mold is that high hardness materials such as tool steel are generally very difficult to machine. As a result, known high-throughput injection molds have long machining times and expensive machining equipment for forming, as well as expensive and time-consuming machining to relieve stress and optimize material hardness A post process is required.

The embodiments shown in the drawings are illustrative and exemplary in nature and are not intended to limit the scope defined by the claims. The following details of exemplary embodiments can be understood when read in conjunction with the following drawings, in which like structure is indicated with like reference numerals.
1 is a schematic diagram of an injection molding machine constructed in accordance with the present disclosure. One Embodiment of the thin wall components formed with the injection molding machine of FIG. 2 is a graph of cavity pressure versus time for the injection molding machine of FIG. Sectional drawing of one Embodiment of the metal mold | die of the injection molding machine of FIG. The perspective view of a supply system. The top view and front view of a natural balance supply system. The top view and front view of a natural balance supply system. The top view and front view of another natural balance supply system. The top view and front view of another natural balance supply system. FIG. 2 is a plan view of an artificial balance supply system that can be used in the injection molding machine of FIG. 1. FIG. 2 is a top view of a non-equilibrium supply system that can be used with the injection molding machine of FIG. 1. FIG. 2 is a top view of a non-equilibrium supply system that can be used with the injection molding machine of FIG. 1.

  Embodiments of the present invention generally relate to systems, machines, products, and methods for producing products by injection molding, and more specifically to systems, products, and methods for producing products by low constant pressure injection molding.

  The term “low pressure” as used herein with respect to the melt pressure of a thermoplastic material means a melt pressure near the nozzle of an injection molding machine of about 41.4 MPa (6000 psi) or less.

  The term “substantially constant pressure” as used herein with respect to the melt pressure of a thermoplastic material means that deviation from the baseline melt pressure does not result in a significant change in the physical properties of the thermoplastic material. To do. For example, “substantially constant pressure” includes, but is not limited to, pressure changes that do not significantly change the viscosity of the molten thermoplastic material. The term “substantially constant” in this regard includes a deviation of up to about 30% from the baseline melt pressure. For example, the term “substantially constant pressure of about 31.7 MPa (4600 psi)” ranges from about 41.4 MPa (6000 psi) (30% over 31.7 MPa (4600 psi)) to about 22.1 MPa (3200 psi) (31 Pressure fluctuations in the range of .7 MPa (4600 psi) 30% below). The melt pressure is considered to be substantially constant as long as the melt pressure varies by only 30% from the stated pressure.

  Fill balance is the term used to define the flow balance of a given plastic because it is dynamically distributed across the injection mold system. The plastic dispensing system includes a hot or cold runner system, as well as a cavity. The injection mold system can be a natural, artificial, or non-equilibrium injection mold system.

  Fill balance is measured by the weight difference between cavities and indicates the performance of a hot or cold runner system. Here, a well-runner runner system is measured by how uniformly the polymer fills each individual cavity, and a perfect runner system is one that fills each cavity at exactly the same time. To do. In conventional injection molding, it is important to balance the flow with respect to the respective cavities, otherwise variations between parts may be large and process capability may not be achievable. Molds with an acceptable flow balance reduce part weight variation, dimensional variation, and shrinkage across any cavity in the mold.

  The level of imbalance is based on a comparison of the weight of all individual parts in the mold measured against the average weight of all cavities in the mold. Measurements are taken when the first cavity to fill reaches 100% fill, the injection process stops, and all parts are weighed to compare weight ranges relative to each other. The imbalance between cavities is calculated relative to the average part weight. The following formula shows a calculation formula for calculating the imbalance using this method.

  An acceptable filling balance is one in which all part weights are typically typically within ± 10% of the average, more desirably ± 5%, and ideally within ± 1%.

  Hot runner design is an important factor for conventional robust injection molding processes and mold design. A poor quality hot runner design can lead to excessive component defects and additional component costs. In order to achieve natural equilibrium using conventional molding processes, the material must pass through the same runner shape from the machine nozzle to each gate. This means not only the same flow distance, but also the same inner diameter and the same number of revolutions along the flow path. The design of these balanced hot runner systems is typically based on common design principles: 1) The pressure drop across the hot runner system is preferably less than 41.4 MPa (6,000 psi) and 2) preferably The volume of molten plastic material contained in the hot runner system does not exceed 3 times the volume contained in the sum of all mold cavities, and 3) the shape of the hot runner flow branch is Optimized to remove dead spots or areas where molten plastic is trapped and cannot flow through the system. These principles often result in oversized inner diameters to achieve a defined pressure drop requirement, and this pressure limitation can lead to a higher ratio of runner volume to part volume than may be desired.

  Constant pressure injection molding allows the hot runner system to be filled at substantially lower pressures than conventional injection molding, and achieves a pressure drop of less than 41.4 MPa (6000 psi) while spanning the hot runner system. Allows the use of smaller and more consistent inner diameters. For example, for low pressure molding where the machine nozzle pressure is less than about 68.9 MPa (10,000 psi), or even more preferably less than about 41.4 MPa (6,000 psi), less than about 20.7 MPa (3,000 psi) Or, more preferably, to achieve a pressure drop across the manifold of less than 13.8 MPa (2,000 psi), or even more preferably less than about 6.89 MPa (1,000 psi). This allows the ability to achieve a substantially lower ratio of runner volume to mold cavity volume, provides fewer material dead spots at runner branch diameter intersections, and allows improved flow balance. Furthermore, as the runner volume decreases, only a small amount of heat is required in the runner system to maintain the molten polymer at the desired processing temperature. In the case of artificial and natural equilibrium feeding systems, a constant pressure treatment is 90% or even higher, even when material viscosity differences are introduced as a result of material batch variations, melt temperature variations, or mold temperature variations. As described above, preferably, a very good filling balance such as a mold balance of 95% or more is maintained. When considering the inner diameter, it is understood that runner channels can be formed using channels of any cross-sectional area and corresponding cross-sectional shape. However, cylindrical runner channels are commonly used to facilitate channel manufacturing simplicity and to minimize frictional forces on the molten polymer.

  Referring to the drawings in detail, FIG. 1 illustrates an exemplary low constant pressure injection molding apparatus 10 (eg, a class 101 or 102 injection mold, or “ultra high productivity mold”) for producing thin wall parts in large quantities. ). The injection molding apparatus 10 generally includes an injection system 12 and a clamping system 14. Thermoplastic material may be introduced into the injection system 12 in the form of thermoplastic pellets 16. The thermoplastic pellets 16 may be placed in a hopper 18 that supplies the thermoplastic pellets 16 to the heating barrel 20 of the injection system 12. The thermoplastic pellets 16 may be moved to the end of the heating barrel 20 by the reciprocating screw 22 after being supplied to the heating barrel 20. By heating the heating barrel 20 with the reciprocating screw 22 and compressing the thermoplastic pellets 16, the thermoplastic pellets 16 are melted to form a molten thermoplastic material 24. The molten thermoplastic material is typically processed at a temperature of about 130 ° C to about 410 ° C.

  The reciprocating screw 22 pushes the molten thermoplastic material 24 toward the nozzle 26 to form a shot of thermoplastic material that is injected into the mold cavity 32 of the mold 28. The molten thermoplastic material 24 may be injected through the gate 30, which induces the flow of the molten thermoplastic material 24 into the mold cavity 32. A mold cavity 32 is formed between the first and second mold parts 25, 27 of the mold 28, and the first and second mold parts 25, 27 are pressed by a press or clamping unit 34. Held together under pressure. A press or clamping unit 34 is used during the molding process to hold the first and second mold parts 25, 27 together while the molten thermoplastic material 24 is being injected into the mold cavity 32. A clamping force is applied in the range of 6.89 MPa (1000 psi) to about 41.4 MPa (6000 psi). To support these clamping forces, the clamping system 14 may include a mold frame and a mold base, the mold frame and mold base being greater than about 165 BHN, as discussed further below. , Preferably formed from a material having a surface hardness of less than 260 BHN, but materials having a surface hardness BHN value greater than 260 can be used as long as the material can be easily machined.

  Once a shot of molten thermoplastic material 24 has been injected into mold cavity 32, reciprocating screw 22 stops moving forward. The molten thermoplastic material 24 takes the form of a mold cavity 32, and the molten thermoplastic material 24 is cooled in the mold 28 until the thermoplastic material 24 solidifies. Once the thermoplastic material 24 has solidified, the press 34 releases the first and second mold parts 25, 27, and the first and second mold parts 25, 27 separate from each other and the final part. Can be removed from the mold 28. The mold 28 may include a plurality of mold cavities 32 to increase the overall production rate. The shape of the cavities of the plurality of mold cavities may be the same, similar, or different from each other (the latter is a group of mold cavities).

  A controller 50 is communicatively connected to the sensor 52 and screw control 36. The controller 50 may include a microprocessor, memory, and one or more communication links. The controller 50 may be connected to the sensor 52 and screw control 36 via each of the wired connections 54, 56. In other embodiments, the controller 50 is a wireless connection, a mechanical connection, a hydraulic connection, a pneumatic connection, or any known to those skilled in the art that allows the controller 50 to communicate with both the sensor 52 and the screw control 36. It may be connected to the sensor 52 and screw control 56 via other types of communication connections.

  In the embodiment of FIG. 1, the sensor 52 is a pressure sensor that measures (directly or indirectly) the melt pressure of the molten thermoplastic material 24 in the nozzle 26. The sensor 52 generates an electrical signal that is transmitted to the controller 50. Controller 50 then commands screw control 36 to advance screw 22 at a rate that maintains a substantially constant melt pressure of molten thermoplastic material 24 in nozzle 26. While the sensor 52 can directly measure the melt pressure, the sensor 52 can measure other properties of the molten thermoplastic material 24, such as temperature, viscosity, flow rate indicative of the melt pressure. Similarly, the sensor 52 need not be located directly in the nozzle 26, but rather the sensor 52 may be at any location within the injection system 12 or mold 28 that is in fluid connection with the nozzle 26. If the sensor 52 is not located in the nozzle 26, an appropriate correction factor can be applied to the measured characteristic to calculate the melt pressure in the nozzle 26. In still other embodiments, sensor 52 need not be fluidly connected to the nozzle. Rather, the sensor may measure the clamping force generated by the clamping system 14 at the mold parting line between the first and second mold parts 25, 27. In one aspect, the controller can maintain the pressure according to input from the sensor.

  A closed loop controller 50 in operation is shown in FIG. 1, but other pressure regulating devices may be used in place of the closed loop controller 50. For example, a pressure adjustment valve (not shown) or a pressure relief valve (not shown) may replace the controller 50 to adjust the melt pressure of the molten thermoplastic material 24. More specifically, the pressure adjusting valve and the pressure relief valve can prevent the mold 28 from being overpressurized. Another alternative mechanism for preventing over pressurization of mold 28 is to activate a warning when an over pressurization condition is detected.

  Referring now to FIG. 2, an exemplary molded part 100 is shown. The molded part 100 is a thin wall part. Molded parts are generally considered to be thin-walled when the flow path length L divided by the flow path thickness T exceeds 100 (ie, L / T> 100). In some injection molding industries, thin wall parts are defined as parts having L / T> 200 or L / T> 250. The channel length L is measured from the gate 102 to the channel end 104. Thin wall components are particularly prevalent in the consumer product manufacturing industry.

  Molded parts are generally considered to be thin-walled when the flow path length L divided by the flow path thickness T exceeds 100 (ie, L / T> 100). For mold cavities with more complex shapes, integrate the T dimension over the length of the mold cavity 32 from the gate 102 to the end of the mold cavity 32 and flow from the gate 102 to the end of the mold cavity 32 By determining the longest flow length of L, the L / T ratio can be calculated. The L / T ratio can then be determined by dividing the longest flow length by the average part thickness.

  Thin wall parts present certain obstacles in injection molding. For example, the thinness of the flow path tends to cool the molten thermoplastic material before the material reaches the flow path end 104. When this occurs, the thermoplastic material freezes and no longer flows, resulting in incomplete parts. In order to overcome this problem, conventional injection molding machines typically have a 103.4 MPa (15%) so that the molten thermoplastic material rapidly fills the mold cavity before the opportunity to cool and freeze. The molten thermoplastic material is injected at a very high pressure in excess of 1,000 psi). This is one reason for teaching thermoplastic material manufacturers to inject at very high pressures. Another reason that conventional injection molding machines inject at high pressure is an increase in shear force, which increases the flow characteristics, as described above. These very high injection pressures require the use of very hard materials to form the mold 28 and delivery system.

  Conventional injection molding machines use tool steel or other hard materials to make the mold. While these tool steels are robust enough to withstand very high injection pressures, tool steels are relatively poor thermal conductors. As a result, a very complex cooling system is machined into the mold to enhance the cooling time when the mold cavity is filled, which reduces cycle time and increases mold productivity Let However, these very complex cooling systems add significant time and expense to the mold making process.

  We have found that shear-thinning thermoplastics (even minimally shear-thinning thermoplastics) are injected into the mold 28 at a low, substantially constant pressure without any significant adverse effects. I found it to get. A variety of thermoplastic materials may be used in the low substantially constant pressure injection molding process of the present disclosure. In one embodiment, the molten thermoplastic material has a melt flow index of about 0.1 g / 10 min to about 500 g / 10 min as measured by ASTM D1238 performed at a temperature of about 230 ° C. and a weight of 2.16 kg. Has a defined viscosity. For example, for polypropylene, the melt flow index can be in the range of about 0.5 g / 10 min to about 200 g / 10 min. Other suitable melt flow indices include from about 1 g / 10 min to about 400 g / 10 min, from about 10 g / 10 min to about 300 g / 10 min, from about 20 to about 200 g / 10 min, from about 30 g / 10 min to about 100 g / 10 min, about 50 g / 10 min to about 75 g / 10 min, about 0.1 g / 10 min to about 1 g / 10 min, or about 1 g / 10 min to about 25 g / 10 min. The MFI of the material is selected based on the application and use of the molded article. For example, a thermoplastic material having an MFI from 0.1 g / 10 min to about 5 g / 10 min may be suitable for use as a preform for injection stretch blow molding (ISBM) applications. A thermoplastic material having a MFI from 5 g / 10 min to about 50 g / 10 min may be suitable for use as a cap and closure for packaging articles. A thermoplastic material having an MFI from 50 g / 10 min to about 150 g / 10 min may be suitable for use in the production of buckets or baskets. A thermoplastic material having a MFI from 150 g / 10 min to about 500 g / 10 min may be suitable for molded articles having a very high L / T ratio, such as sheet. Manufacturers of such thermoplastic materials generally inject these materials with a melt pressure greater than 41.4 MPa (6000 psi), and often much greater than 41.4 MPa (6000 psi). Teach it to be molded. Contrary to conventional teachings regarding injection molding of such thermoplastic materials, the low constant injection molding embodiment of the present disclosure uses such thermoplastic materials to form high quality injection molded parts. And possibly with a melt pressure below 41.4 MPa (6000 psi), and in some cases well below 41.4 MPa (6000 psi).

  The thermoplastic material may be, for example, a polyolefin. Exemplary polyolefins include, but are not limited to, polypropylene, polyethylene, polymethylpentene, and polybutene-1. Any of the aforementioned polyolefins can be procured from bio-based raw materials such as sugar cane or other agricultural products to produce biopolypropylene or biopolyethylene. Polyolefins advantageously exhibit shear thinning when in the molten state. Shear thinning is the decrease in viscosity when a fluid is placed under compressive stress. Shear thinning can advantageously allow the flow of thermoplastic material to be maintained throughout the injection molding process. While not intending to be bound by theory, it is believed that the shear thinning properties of thermoplastic materials, specifically polyolefins, result in less variation in material viscosity when the material is processed at low pressure. Conceivable. As a result, embodiments of the disclosed method may be less sensitive to variations in thermoplastic materials due to, for example, colorants and other additives and processing conditions. This reduced sensitivity to batch-to-batch variations in thermoplastic material properties may also advantageously allow deindustrialized and used recycled plastics to be processed using the method embodiments of the present disclosure. De-industrialized and used recycled plastic comes from end products that have finished their life cycle as consumer goods and are otherwise discarded as solid waste. Such recycled plastics and blends of thermoplastic materials inherently have significant batch-to-batch variations in their material properties.

  The thermoplastic material may be, for example, polyester. Exemplary polyesters include, but are not limited to, polyethylene terephthalate (PET). PET polymers can be extracted from bio-based raw materials such as sugar cane or other agricultural products to partially or fully produce bio-PET polymers. Other suitable thermoplastic materials include polypropylene and polyethylene copolymers, and polymers and copolymers of thermoplastic elastomers, polyesters, polystyrenes, polycarbonates, poly (acrylonitrile-butadiene-styrene), poly (lactic acid), biobased polyesters. For example, poly (ethylene furanoate) polyhydroxyalkanoate, poly (ethylene furanoate) (considered as a substitute or drop-in substitute for PET), polyhydroxyalkanoate, polyamide, polyacetal, ethylene alpha olefin rubber, and Styrene butadiene styrene block copolymers are included. The thermoplastic material can also be a blend of a plurality of polymeric and non-polymeric materials. The thermoplastic material can be, for example, a blend of macromolecules, intermediate molecules, and low molecular weight polymers resulting in a diversity or bimodal blend. The versatile material may be designed in a way that results in a thermoplastic material that has excellent flow properties and also has sufficient chemical / physical properties. The thermoplastic material can also be a polymer blend with one or more small molecule additives. The small molecule can be, for example, a siloxane or other lubricating molecule that improves the fluidity of the polymeric material when added to a thermoplastic material.

  Other additives include inorganic fillers such as calcium carbonate, calcium sulfate, talc, clay (e.g. nanoclay), aluminum hydroxide, CaSiO3, glass formed on fibers or microspheres, crystalline silica (e.g. (Quartz, novasite, crystallobite), magnesium hydroxide, mica, sodium sulfate, lithopone, magnesium carbonate, iron oxide, or organic fillers such as rice husk, straw, hemp fiber, wood flour, or wood, bamboo, or sugar cane Fibers can be included.

  Other suitable thermoplastic materials include renewable polymers, such as those produced directly from an organism, such as polyhydroxyalkanoates (eg, poly (β-hydroxyalkanoate), poly (3- Hydroxybutyrate-co-3-hydroxyvalerate, NODAX®) and bacterial cellulose; macromolecules extracted from plants, agricultural forests, and biomass, such as polysaccharides and derivatives thereof (eg, gums, Cellulose, cellulose ester, chitin, chitosan, starch, chemically modified starch, cellulose acetate particles), proteins (eg, zein, whey, gluten, collagen), lipids, lignin, and natural rubber; produced from starch or chemically modified starch Thermoplastic starch; and naturally supplied mono -Non-limiting examples of current polymers derived from and derivatives such as biopolyethylene, biopolypropylene, polytrimethylene terephthalate, polylactic acid, NYLON 11, alkyd resin, succinic polyester, and biopolyethylene terephthalate. included.

  Suitable thermoplastic materials may include one blend or multiple blends of different thermoplastic materials such as those described above. The different materials can also be a combination of materials derived from unused bio-derived or petroleum-derived materials, or recycled materials of bio-derived or petroleum-derived materials. One or more of the thermoplastic materials in the blend can be biodegradable. In the case of a non-blended thermoplastic material, the material can be biodegradable.

  Parts molded at low, substantially constant pressure exhibit several superior properties compared to identical parts molded at conventional high pressures. This finding is completely inconsistent with conventional wisdom within the production industry that teaches that higher injection pressures are better. Without being bound by theory, injecting molten thermoplastic material into the mold 28 at a low, substantially constant pressure will advance the heat from the gate through the mold to the farthest part of the mold cavity. It is thought to create a continuous flow front of plastic material. By maintaining a low level of shear, the thermoplastic material remains liquid and flowable at much lower temperatures and pressures than would otherwise be possible with conventional high pressure injection molding systems. is there.

  Referring now to FIG. 3, a typical pressure-time curve for a conventional high pressure injection molding process is shown by dashed line 200. In contrast, the pressure-time curve for the disclosed low constant pressure injection molding machine is shown by the solid line 210.

  In the conventional case, the melt pressure increases rapidly to well above 103.4 MPa (15,000 psi), and then at a relatively high pressure above 103.4 MPa (15,000 psi) over the first period 220. Retained. The first period 220 is the filling time for the molten plastic material to flow into the mold cavity. Thereafter, the melt pressure decreases and remains at a lower but still relatively high pressure of 68.9 MPa (10,000 psi) over the second period 230. The second period 230 is the packing time during which the melt pressure is maintained to ensure that the entire gap in the mold cavity is backfilled. The mold cavity in the conventional high pressure injection molding system is filled back from the end of the flow path toward the gate. As a result, the plastics in the various solidification stages can be packed on top of each other and cause inconsistencies in the final product, as described above. Furthermore, conventional packing of plastics at various stages of solidification provides several non-ideal material properties such as residual molding stress, sink, sink, non-optimal optional properties.

  On the other hand, a constant low pressure injection molding system injects molten plastic material into the mold cavity at a substantially constant low pressure over a single period 240. The injection pressure is typically less than 41.4 MPa (6,000 psi). By using a substantially constant low pressure, the molten thermoplastic material maintains a continuous melt front that advances through the flow path from the gate toward the end of the flow path. Thus, the plastic material remains relatively uniform at any point along the flow path, which results in a more uniform and consistent final product. By filling the mold with a relatively uniform plastic material, the final molded part forms a crystalline structure with better mechanical and optical properties than conventional molded parts. Furthermore, the skin layer of a part molded at a low constant pressure exhibits different properties than the skin layer of a conventional molded part. As a result, the skin layer of a part molded at a low constant pressure can have better optical properties than the skin layer of a conventional molded part.

  Form mold 28 and / or feed system with more machinable material by maintaining a substantially constant and low melt pressure (e.g., less than 6000 psi) within the nozzle. be able to. For example, the mold 28 shown in FIG. 1 has a milling machining index greater than 100%, a drilling machining index greater than 100%, a wire EDM machining index greater than 100%, and a graphite sinker EDM machining index greater than 200%. Or a material having a copper sinker EDM machining index greater than 150%. The machining index is based on milling, drilling, wire EDM, and sinker EDM tests of various materials. The test method for determining the machining index is described in more detail below. Examples of machining indices for material samples are summarized in Table 1 below.

  The use of easily machinable materials to form the mold 28 results in greatly reduced manufacturing time and thus lower manufacturing costs. Furthermore, these machinable materials generally have better thermal conductivity than tool steel, which increases cooling efficiency and reduces the need for complex cooling systems.

When forming these readily machinable material molds 28, it is also advantageous to select an easily machinable material that has good thermal conductivity properties. Particularly advantageous are materials having a thermal conductivity in excess of 51.9 W / (m ^* K) (30 BTU / HR FT ° F). For example, easily machineable materials with good thermal conductivity include, but are not limited to, Alcoa QC-10, Alcan Duramold 500, and Hokotol (available from Aleris). A material with good thermal conductivity will more efficiently transfer heat from the thermoplastic material out of the mold. As a result, a simpler cooling system can be used. In addition, non-natural equilibrium feeding systems can also be used with certain low pressure injection molding machines described herein.

  An example of a multi-cavity mold 28 is illustrated in FIGS. 4A and 4B. Multi-cavity molds generally include a supply manifold 60 that directs molten thermoplastic material from nozzles 26 to individual mold cavities 32. The supply manifold 60 includes a sprue 62 that directs molten thermoplastic material to one or more runners or supply channels 64. Each runner may be supplied to a plurality of mold cavities 32. In many high volume injection molding machines, the runner is heated to enhance the fluidity of the molten thermoplastic material. Because the viscosity of the molten thermoplastic material is very sensitive to shear forces and pressure fluctuations at high pressures (eg, greater than 68.9 MPa (10,000 psi)), conventional supply manifolds have uniform viscosity. To maintain the natural equilibrium. The natural equilibrium supply manifold is a manifold in which the molten thermoplastic material moves equidistant from the gate to an arbitrary mold cavity. Furthermore, the cross-sectional shape of each flow path is the same, the number and kind of bends are the same, and the temperature of each flow path is the same. The natural balance supply manifold allows the mold cavities to be filled simultaneously so that each molded part has the same processing conditions and material properties.

  FIG. 5 illustrates one embodiment of a natural equilibrium supply manifold 60. The natural equilibrium supply manifold 60 includes a first flow path 70 from the gate 62 to the first joint 72, where the first flow path 70 is connected to the second and third flow paths 74, 76. The second flow path terminates at a second gate 78a, the third flow path 76 terminates at a third gate 78b, and each gate is an individual mold cavity (shown in FIG. 5). Supply). The molten thermoplastic material flowing from the gate 62 to either the second gate 78a or the third gate 78b travels the same distance, experiences the same temperature, and is subjected to the same cross-sectional flow area. As a result, each mold cavity is filled simultaneously with a molten thermoplastic material having the same physical properties.

  6A and 6B schematically illustrate a natural balance manifold 60. The natural balance manifold 60 of FIGS. 6A and 6B is a multi-layer manifold. Each flow path 74, 76 has the same characteristics at the same position along the flow path. For example, after the joint portion 72, each flow path becomes narrow at the same distance. Furthermore, each flow path supplies the same number of mold cavities 32. The natural equilibrium flow manifold 60 is essential for high pressure injection molding machines to maintain the same plastic flow characteristics and to ensure uniform parts.

  7A and 7B show another natural balance manifold 60. The natural balance manifold 60 of FIGS. 7A and 7B is a single layer manifold.

  In contrast, FIGS. 8, 9A, and 9B show a non-natural equilibrium manifold, FIG. 8 shows an artificial equilibrium manifold, and FIGS. 9A and 9B show a non-equilibrium manifold.

  The low constant pressure injection molding machine disclosed herein is an artificial balance manifold because the thermoplastic material injected at a low constant pressure is not very sensitive to pressure differences or shear force differences due to differences in flow path characteristics, And even non-equilibrium manifolds can be used. In other words, a thermoplastic material injected at a low constant pressure retains more uniform and balanced material and flow characteristics regardless of differences in channel length, cross-sectional area, or temperature. This provides a substantially more balanced flow in a natural non-equilibrium design and provides more uniform material properties across all part cavities if the flow can be very imbalanced.

  The artificial balance manifold 160 of FIG. 8 includes a gate 62, a first channel 174, and a second channel 176. The first channel 174 terminates at the first gate 178a, and the second channel 176 terminates at the second gate 178b. In the present embodiment, the first flow path 174 is shorter than the second flow path 176. The artificial balance manifold 160 may be configured with a number of other parameters (eg, cross-sectional area or temperature) such that the material flowing through the manifold 160 provides equilibrium flow to each cavity, similar to the natural balance manifold. ). In other words, the thermoplastic material flowing through the first flow path 174 has a melt pressure that is approximately equal to the thermoplastic material flowing through the second flow path 176. Artificial balanced or unbalanced supply manifolds can make much more efficient use of space because artificially balanced or unbalanced supply manifolds can include channels of different lengths. Furthermore, the feed channel and the corresponding heater band channel can be machined more efficiently. Further, the natural balance supply manifold is limited to molds having an even number of different mold cavities (eg, 2, 4, 8, 16, 32, etc.). Artificial balanced or non-equilibrium supply manifolds can be designed to deliver molten thermoplastic material to any number of mold cavities.

  The artificial balance supply manifold 160 may also be constructed from a material with high thermal conductivity to enhance heat transfer to the molten thermoplastic material in the hot runner and thus enhance the flow of the thermoplastic material. More specifically, the artificial balance supply manifold 160 may be constructed from the same material as the mold to further reduce material costs and enhance heat transfer within the overall system.

  9A and 9B show a non-equilibrium manifold 260. FIG. The non-equilibrium manifold 260 may include an odd number of mold cavities 232 and / or channels having different cross-sectional shapes, different numbers and types of bends, and / or different temperatures. Further, the non-equilibrium manifold 260 may provide mold cavities having different dimensions and / or different shapes illustrated in FIG. 9B, or mold cavities that are oriented differently relative to a common surface of the mold. . Further, the non-equilibrium manifold 260 may supply an injection mold that includes an injection mold having 8 or more mold cavities and / or an inductive extraction system.

  In still other embodiments, an artificial balanced manifold and / or a non-equilibrium manifold may be used in a mold having mold cavities in separate layers, such as a stack mold configuration, where the manifold is one of the stack molds. Supply mold cavities in two or three layers. The mold cavity may receive molten plastic from more than one gate, or each individual gate may deliver more than one material in succession to an individual mold cavity. Further, two or more manifolds may provide a cavity position that rotates within the mold from a first position where the first material is introduced to a second position where the second material is introduced. Good.

  Furthermore, the non-natural equilibrium supply system described above may include one or more heating elements in thermal communication with one of the supply channels. The heating element is contained in a material having substantially the same thermal conductivity as the material forming the majority of the mold or a material having substantially the same thermal conductivity as the material forming the delivery system. Also good.

Drilling and Milling Machinability Index Test Method The drilling and milling machinability index listed in Table 1 above was determined by testing representative materials in the carefully controlled test method described below.

  Determine the machinability index for each material by measuring the spindle load required to drill or grind a piece of material, and all other machine conditions (eg, stock feed rate, spindle rpm, etc.) Was held constant between the various materials. Spindle load is reported as the ratio of the measured spindle load to the maximum spindle torque load of 101.7 Nm (75 ft-lb) at 1400 rpm for a drilling or milling device. The index percentage was calculated as the ratio between the spindle load for 1117 steel and the spindle load for the test material.

  The test milling or drilling machine was a Hass VF-3 machining center.

  “Flood blast” cooling was used for all tests. The coolant was Koolrite 2290.

EDM Machinability Index Test Method The graphite and copper sinker EDM machinability index listed in Table 1 above was determined by testing representative materials in the carefully controlled test method described below.

  The EDM machinability index for various materials was determined by measuring the time to bake ranges (details below) on various test metals. The machinability index percentage was calculated as the ratio of the time to bake 1117 steel to the time required to bake the same range to other test materials.

  The disclosed low constant pressure injection molding machine advantageously utilizes a mold constructed from an easily machineable material. As a result, the disclosed low constant pressure injection molding machine is cheaper and faster to manufacture. In addition, the disclosed low constant pressure injection molding machine has a wider platen width, increased tie bar spacing, tie bar removal, lighter weight structure to facilitate faster movement, and non-natural balance feeding system, etc. It is possible to utilize a more flexible support structure and a more flexible delivery structure. Thus, the disclosed low constant pressure injection molding machine can be improved to meet delivery needs and can be more easily customized for specific molded parts.

  Unless otherwise specified, the terms “substantially”, “about”, and “approximately” are used herein to belong to any quantitative comparison, value, measurement, or other expression. Note that can be used to represent uncertain specificity. These terms are also used herein to describe the extent to which the quantitative expression varies from the stated criteria without causing a change in the basic function of the object in question. Unless otherwise specified herein, the terms “substantially”, “about”, and “approximately” refer to quantitative comparisons, values, measurements, or other expressions that are 20 It means that it can enter within%.

  It will now be apparent that various embodiments of the products illustrated and described herein may be made by a low constant pressure injection molding process. In this specification, reference was made in particular to products containing consumer goods or the consumer goods products themselves. Obviously, it may be suitable for use with products used in, etc. Further, those skilled in the art will be able to combine the teachings disclosed herein with in-mold decorations, insert molding, in-mold assemblies, and the like, including multiple molds, including stack molds, rotating and core back molds. Understand that it can be used in mold construction. Further, those skilled in the art will be able to combine the teachings disclosed herein with in-mold decorations, insert molding, in-mold assemblies, and the like, including multiple molds, including stack molds, rotating and core back molds. Understand that it can be used in mold construction.

  Virtual modeling programs such as Sigmasoft and Moldflow can be used to predict the pressure, fill rate, and cooling time required to fill the mold cavity. These programs can model processes that are controlled by polymer flow, pressure, or a combination of flow and pressure. These programs are used in runner design, gate location, and mold design.

  All references cited in “Modes for Carrying Out the Invention” of the present invention are hereby incorporated by reference in the relevant part, and any citation of any reference is accepted as prior art to the present invention. Should not be construed as doing. To the extent that any meaning or definition of a term in this document contradicts any meaning or definition of a term in a document incorporated by reference, the meaning or definition given to that term in this document applies. Shall be.

  While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications can be made without departing from the spirit and scope of the claims. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects may not be utilized in combination. Accordingly, the appended claims are intended to include all such changes and modifications as fall within the scope of the claims.

Claims (4)

A non-equilibrium supply system for an injection mold of an injection molding apparatus, wherein the injection mold has a plurality of mold cavities, the plurality of mold cavities receive molten plastic, and the injection molding apparatus comprises:
A first supply channel that terminates in a first mold cavity of the plurality of mold cavities; a second supply channel that terminates in a second mold cavity of the plurality of mold cavities; A hot runner having the non-equilibrium design of the first supply channel and the second supply channel;
A control system for injecting the molten plastic into the mold cavity with a nozzle pressure that is less than 68.9 MPa and fluctuates less than 30%, the supply system maintaining a mold balance of at least 95% A non-equilibrium feeding system comprising: a control system, wherein the variation in part weight of all mold cavities is less than 5%.   The length of the first supply channel is different from the length of the second supply channel, so that the first supply channel and the second supply channel have an unbalanced design. The described non-equilibrium supply system.   The temperature of the molten plastic flowing through the first supply channel is different from the temperature of the molten plastic flowing through the second supply channel, so that the first supply channel and the second supply channel are in an unbalanced design. The non-equilibrium supply system according to claim 1.   The non-equilibrium supply system of claim 1, wherein the control system injects the molten plastic into the mold cavity such that the component weight variation of all mold cavities is less than 1%. .

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